The invention relates to a measuring circuit and a measuring method for monitoring a component of an electrical assembly, as well as an electrical assembly, in particular a switching power supply, having such a measuring circuit to execute the measuring method.
In switching power supplies, capacitors are used on both the primary and secondary sides to smooth and stabilize voltages, in particular rectified AC voltages. Usually, electrolytic capacitors are used for this purpose, since these include high capacities and low component weights, and since they are relatively low-cost. The capacity of capacitors, in particular of such electrolytic capacitors, decreases as the number of operating hours increases. This usually goes along with an increase of their inner resistance, also referred to as equivalent series resistance (ESR).
Frequently, these smoothing and stabilizing capacitors are the cause for failure of a switching power supply. They significantly compromise and limit the lifespan of the switching power supply. The aging process of the capacitors generally is not linear. Usually, an initially low, relative reduction in capacity or low, relative increase in the equivalent series resistance increases over time. Once the capacity falls below, or the equivalent series resistance rises above, a certain threshold, the switching power supply is no longer operable or can only operate at low loads. Detecting an imminent failure of the switching power supply before the switching power supply actually fails requires being able to determine either the reduction in capacity or the increase in equivalent series resistance with sufficient accuracy, such that even small deviations from the baseline can be detected.
Aside from the capacitors, the semiconductor switches of the switching power supply, in particular the clocked semiconductor switch used on the primary side of the switching converter, represent a frequent cause for the failure of a switching power supply. Typically, field-effect transistors are used as clocked semiconductor switches, in particular metal oxide semiconductor field-effect transistors (MOSFETs).
Due to carrier diffusion processes and/or boundary condition changes caused by consistently high temperatures, the effective resistance of the field-effect transistor in its conductive state changes over time. An increase of this effective resistance in the conductive state leads to higher power losses in the transistor, which leads to further temperature increases, which in turn accelerates the aging process even further. For this reason, the aging process in field-effect transistors, similar to that in capacitors, is not linear, but self-accelerating. Correspondingly, early detection of changes in the effective resistance of a field-effect transistor in its conductive state is also desirable, in order to be able to predict imminent failure in due time, before failure actually occurs.
Both the determination of the effective resistance of a semiconductor switch or equivalent series resistance of a capacitor and the determination of the capacity of the capacitor are based on a measurement of current and voltage. In the determination of capacity, the current flowing in a capacitor is linked to the change in voltage across the capacitor via the capacity of the capacitor. In the determination of the equivalent series resistance of the capacitor or of the effective resistance of the semiconductor switch, respectively, the resistance is the quotient of the applied voltage and the flowing current.
The previously described determinations of equivalent series resistance, effective resistance, or capacity therefore require a measurement of voltage and current for each component to be monitored. Switching power supplies are operated at clock frequencies in the range of tens of kHz (kilo-Hertz), up to 100 kHz. Changes in this clock frequency cause changes in the voltages and currents in the components to be monitored. As these measured values change periodically with the clock frequency of the switching power supply, the method according to the state of the art is to simultaneously measure the voltage and current values to be used in the calculations. This requires one measuring device per component, for example, one A/D (analog/digital) converter for the voltage and another one for the value of the current. A/D converters, with suitable specifications regarding their speeds, are cost-intensive components. For example, monitoring the primary-side capacitor, the primary-side semiconductor switch and two secondary-side capacitors in a switching power supply would require two such A/D converters per respective component, for a total requirement of eight such converters.